The present invention relates to an internal combustion engine fuel injector, and to a relative method of classifying and selecting a series of injectors.
Known fuel injectors generally comprise a nozzle normally closed by a rod which is caused to slide inside a cylindrical guide by the fuel pressure in a control chamber having a calibrated fuel intake conduit and a calibrated discharge conduit for discharging the chamber.
The discharge conduit is controlled by a metering valve in turn controlled by an electromagnet; and the length of time the electromagnet is energized varies according to the amount of fuel to be injected to achieve a given power of the engine. Depending on engine speed, the maximum power obtainable varies according to the so-called xe2x80x9cpower curvexe2x80x9d of the engine.
The pressurized fuel for injection is fed along a feed conduit to an injection chamber located at a pin cooperating with the rod and engaging the injection orifices in the nozzle; and the injection chamber and feed conduit are normally sized to ensure the maximum amount of fuel is injected as quickly as possible.
When the electromagnet is energized, the metering valve is opened to move the rod into a stop position.
More specifically, in known so-called xe2x80x9chydraulic stopxe2x80x9d injectors, the rod is arrested, dynamically balanced, a given distance from an end wall of the cylindrical rod guide, after a travel or lift which is roughly 0.2-0.25 mm for unit displacement engines up to 0.65 liters/cylinder, and up to 0.4 mm for higher displacement engines.
The behaviour of an injector of the above type is normally represented by a characteristic defining the amount of fuel injected as a function of the length of time the electromagnet is energized, and which is typically defined by a broken line comprising two substantially straight portions sloping at different angles. More specifically, the second portion slopes at a smaller angle than the first, and the two portions form a so-called xe2x80x9ckneexe2x80x9d at the point at which the rod, as it moves towards the stop position, is arrested in dynamic equilibrium.
Since mass produced injectors perform differently, the individual injectors fitted to a given engine must be tested and classified according to the delay (offset) between the start of injection and the instant the electromagnet is energized, and according to behaviour during injection, to define the amount of fuel injected alongside variations in the length of time the injector is opened. To reduce the difference in the amount of fuel injected into the engine cylinders by the respective injectors, each engine is fitted with injectors of the same class.
Known injectors have several drawbacks. In particular, the varying increase in the amount of fuel injected is accompanied by inconsistent behaviour of the injector, which varies widely from one injector to another and depends on the various conduit machining tolerances and mechanical connections. Classifying injectors is therefore a painstaking or random business, also on account of the hydraulic disturbance caused by arrest of the rod and the consequent reduction in the increase in the amount of fuel injected.
It is an object of the present invention to provide a fuel injector of the above type, which is extremely straightforward and reliable, and can be classified and selected accurately for assembly to a given engine, so as to eliminate the aforementioned drawbacks typically associated with known injectors.
According to the present invention, there is provided a fuel injector for an internal combustion engine having a predetermined power curve; the injector comprising a nozzle normally closed by a rod; said rod being movable, from a closed position closing said nozzle and through an opening stroke and a closing stroke, by the fuel pressure in a control chamber; said control chamber having an intake conduit and a discharge conduit; a metering valve, comprising a plug for said discharge conduit, being controlled by energizing an electromagnet for a variable length of time corresponding to the amount of fuel for injection; and the injector being characterized in that said rod is movable from said closed position to a position corresponding to a maximum travel, when said electromagnet is energized for a corresponding maximum time interval, so that the amount of fuel injected, between the start of said opening stroke into said position corresponding to said maximum travel, and the end of the respective closing stroke, is greater than a maximum amount of fuel required by said power curve.
The method of classifying and selecting a series of fuel injectors according to the invention is characterized by comprising the steps of:
preparing a series of injectors with corresponding rods; each of said rods being movable from a closed position and through an opening stroke to a position corresponding to a maximum travel, so that the amount of fuel injected, between the start of said opening stroke into said position corresponding to said maximum travel, and the end of the respective closing stroke, is greater than a maximum amount of fuel required by a power curve;
testing the amount of fuel injected by each of said injectors, by moving the corresponding rod into two positions at distances, from said closed position, smaller than said maximum travel, so as to define the delay in the start of injection and the increase in the amount of fuel injected; and
selecting for a given engine a group of injectors having a given delay and a given increase in the amount of fuel injected.